I am soon moving to all-grain but can't afford a propane burner and ss mega pot so I will be boiling with two 5gal canning pots. I do 6 gal batches so I plan to collect 8 gallons of wort (4gal by pot) that will boil down to 3 gals each. I understand that the concentration remains the same, since 3gals of 1.050 wort added to another 3gals of 1.050 wort will simply give 6gals of 1.050.

My question is about the hops. Do I have to split the quantity in half for each pot ?? I know the utilisation is a matter of AAU and gravity (and some other factors) so the IBU in each pot will be half of the predicted IBU for the 6 gal batch.

You have two three gallon batches. If, independently their IBU's = 20, then if you combined the two, the IBU's will still be 20 not 40. You could put all the hops in one 3 gallon batch and 0 in the second and get an IBU of 40 in the first batch but when you combine the two you have to divide by 2 = 20 IBU's